Which of the following is not a component of immune system ?
Which of the following is not a component of immune system ?
Options
Correct option: C — Red Blood Cells
Red Blood Cells (erythrocytes) transport oxygen and carbon dioxide; they play no role in immune defence.
- Lymphocytes, plasma proteins (antibodies/complement) and bone marrow are all part of the immune system.
- RBCs lack a nucleus and immune function, so they are the odd one out.
Marking Scheme
- 11 mark: correct option C (Red Blood Cells).
- 2Full mark awarded only for the correct choice.
Hint
Which blood component only carries oxygen and has no defence role?
Quick Oral Answer
Red blood cells are the answer — they only transport respiratory gases; lymphocytes, plasma proteins like antibodies, and bone marrow are all genuine immune-system components.
Analysis & Explanation
Concept
The immune system comprises lymphoid organs, immune cells, tissues and soluble molecules that defend the body against pathogens.
Why C is correct
- Red Blood Cells are respiratory transport cells (carry O2/CO2 via haemoglobin). They are anucleate in mammals and have no defensive/immune role, so they are NOT a component of the immune system.
Why the distractors are wrong
- Lymphocytes — B-cells and T-cells are the central cells of specific (acquired) immunity.
- Plasma Proteins — include antibodies (immunoglobulins) and complement proteins, key humoral defence molecules.
- Bone Marrow — a primary lymphoid organ where all blood cells, including lymphocytes, originate and B-cells mature.
Exam trap
Because RBCs, WBCs and plasma proteins all circulate in blood, students wrongly assume RBCs are immune cells. Only WBCs (leucocytes) and their products participate in immunity.
Common Mistakes
- 1Assuming all blood cells and proteins are immune components — RBCs only transport gases.
- 2Not recognising bone marrow as a primary lymphoid organ where lymphocytes are formed.
- 3Overlooking that plasma proteins include antibodies and complement, which are immune molecules.
Interesting Facts
Bone marrow and the thymus are the two primary lymphoid organs; spleen, lymph nodes and MALT are secondary lymphoid organs.
A single millilitre of blood contains billions of RBCs but they are removed from immune roles entirely — even lacking a nucleus in mammals.
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Frequently Asked Questions
Why are red blood cells not part of the immune system?
RBCs are specialised for transporting oxygen and carbon dioxide using haemoglobin. In mammals they lack a nucleus and have no capacity for phagocytosis, antibody production or antigen recognition, so they are excluded from immune defence.
Is bone marrow really part of the immune system?
Yes. Bone marrow is a primary lymphoid organ. All blood cells, including immune lymphocytes, are produced here, and B-lymphocytes complete their maturation in the bone marrow.